High-vacuum pumps often include molecular pump stages or turbomolecular pump stages. In molecular pumps, a moving rotor wall and a resting stator wall are designed and positioned such that, in operation, they impart predetermined directional forces to convey gas molecules situated therebetween. As a rule, either the rotor wall or the stator wall is equipped with spiral or helical depressions or projections that determine the direction of the forces transmitted, and thus the conveying direction of the gas. Turbomolecular pump stages include stator paddles and rotor paddles arranged to resemble a turbine. The paddles transmit the desired conveying forces onto the gas molecules in a predetermined conveying direction.
Turbomolecular pump stages have a relatively low compression ratio (defined as the ratio of the pressure at the thrust face to the pressure at the suction face) but have a relatively high pumping capacity (pump speed, volume flow per time unit). Their manufacture and assembly, however, is involved and expensive, since a great number of pump stages (rotor and stator stages) are required in order to achieve an adequate compression. Molecular pump stages have a relatively high compression ratio; their pumping capacity, however, is poor.
European Patent Application No. 142 208 discloses that the pumping capacity of a molecular pump can be improved when a separate pump stage is provided at the suction side of the molecular pump. The separate pump stage includes a helical projection at the stator side. This projection is the continuation of a helical projection of the molecular pump in the direction of the suction side. The rotor side of the helical projection includes paddle blades extending radially and parallel to the rotational axis of the rotor. A pump stage of this type is relatively involved to manufacture, since it requires structure both at the rotor side as well as at the stator side. Further, the compression of these pump stages is very low.